1. Technical Field of the Invention
The present invention relates to the field of integrated circuits, and more particularly to mask-programmable memory (MPM).
2. Prior Arts
With a large storage capacity, low cost and excellent security, mask-programmable memory (MPM) is considered as an ideal storage medium for multimedia contents and others. FIGS. 1A-1B illustrate two prior-art MPM cells. Among these, cell 1aa, located at the intersection of upper address-selection line (ASL) 20a and lower ASL 30a, comprises no contact. It has large contact resistance and therefore, represents “0”. On the other hand, cell 1ab comprises a full contact 3ab. It has low contact resistance and represents “1”. These prior-art cells can have only two states: no-contact or full-contact. Thus, each cell can represent only one bit of digital information. As a result, the MPM storage density is limited. To increase the storage density, the present invention discloses an N-ary mask-programmable memory (N-MPM).